Project Summary: The heart consists of a multitude of diverse cardiac cell types, including but not limited to (chambered and non- chambered) cardiomyocytes, cardiac fibroblasts, epicardial cells, endothelial/endocardial cells and smooth muscle cells, which organize into distinct cardiac structures that coordinately regulate proper cardiac function and circulation throughout the body. Because loss or dysfunction of these cell types individually or combinatorially can lead to either adult or congenital heart diseases, increasing efforts have been recently devoted toward understanding how these diverse cardiovascular cell-types are created and function in order to develop potential human cardiac therapies. Such endeavors have illuminated not only the origins of many cardiac lineages but also key signaling cues and transcriptional regulators which in turn have been recently employed to efficiently direct various non-cardiac sources including human pluripotent stem cells and human fibroblasts into specific cardiac cell types for regenerative therapies. However, how cardiac cell types forming the cardiac inflow tract and the developmentally-related epicardium develop remains less certain in part due to their unclear origins. To shed light on these issues, we propose to discover the origins of these unique cardiac cell-types, which we posit derive from a common cardiac progenitor source, and then identify underlying signaling mechanisms that regulate their development. Toward this end, we will examine whether outlying Nkx2.5+ cardiac mesoderm is fated to become cardiac inflow tract cardiomyocytes and epicardial cells, identify signaling pathways that specify cardiac mesoderm into cardiac progenitors which create the cardiac inflow tract myocardium and epicardium and investigate whether these developmental mechanisms are conserved across vertebrates.